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I Largest Hard Planet of Gravity 1g we have ever Discovered?

  1. Sep 3, 2016 #1
    What is the largest planet (in terms of size) with a hard surface that you can walk on and has a surface gravity close to that of Earth that we have ever discovered?

    What about the smallest?
  2. jcsd
  3. Sep 3, 2016 #2

    Simon Bridge

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  4. Sep 3, 2016 #3
    thanks for the links. Gliese 581 d seems to be a good match.

    It'd be cool to explore. Feels like you're walking on Earth but with more than 5 times the amount of area to explore!
  5. Sep 4, 2016 #4


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    The density would need to be quite low, if the variation of density with depth has the same proportions as Earth. Wouldn't that mean the surface could be very soft and weak. Snow shoes might be required and there could be some massive deep craters from impacting meteorites?
  6. Sep 4, 2016 #5
    Venus is quite similar to Earth in terms of size and gravity, and this is accurate measurement.
    For exoplanets, these values are still somewhat guesstimates, but better accuracy is likely with the next generations of telescopes and other instruments.
    Walking on Venus is clearly not an option though with temperatures around 462 °C (863 °F), and a toxic corrosive atmosphere.
    We know next to nothing about surface temperatures and the atmospheres of exoplanets at this stage.
    Last edited: Sep 4, 2016
  7. Sep 4, 2016 #6
    ok. I'm looking for the largest planet though
  8. Sep 6, 2016 #7
    Not so much. Gliese 581d has a radius 2.2 times that of Earth. Assuming it is a terrestrial/rocky exoplanet and not gaseous, it would have an approximate density of 9.888 g/cm3, or 1.795 times that of Earth. Furthermore, Gliese 581d has a mass that ranges from 6.98 to 13.8 that of Earth. If you use the lower estimate that would give Gliese 581d an approximate surface gravity of 68.45 m/s2, or ~7 G. There is also evidence to suggest that exoplanets larger than 1.6 Earth radii are not terrestrial/rocky.

    A better choice might be Kepler 102f. It is a little to close to its star to fall within the habitable zone, but its mass and radius give it an estimated gravity of 0.86 G. So while Kepler 102f would be uncomfortably hot, it might have a surface gravity that terrestrial life would find accommodating.

    Most 1.6 Earth-Radius Planets are not Rocky - The Astrophysical Journal, Volume 801, Number 1, March 2, 2015 (free issue)
    The Mass-Radius Relation for 65 Exoplanets Smaller than 4 Earth Radii - The Astrophysical Journal Letters, Volume 783, Number 1, February 13, 2014 (free issue)
    Last edited: Sep 6, 2016
  9. Sep 14, 2016 #8
    For exoplanets, one either has their projected masses (mass * sin(orbit inclination)) or radii, though sometimes one finds both. You'd need both for getting the planet's surface gravity.

    But I've found some planet-structure calculations at http://seagerexoplanets.mit.edu/ftp/Papers/Seager2007.pdf -- at Sara Seager's site. It has some mass-radius formulas for various compositions. I've chosen rock and iron because they give solid surfaces at room temperature. For 1 Earth mass, these sizes in Earth radii and their surface gravities:
    • Rock (MgSiO3): 1.02, 0.95
    • 70% rock, 30% iron by mass: 0.97, 1.06 -- like the Earth
    • 32.5% rock, 67.5% iron by mass: 0.86, 1.34
    • Iron (alpha phase of solid): 0.79, 1.60

    To get 1 Earth gravity (mass, radius):
    • Rock: 1.1, 1.05
    • 70%-30%: 0.86, 0.93
    • 32.5%-67.5%: 0.48, 0.69
    • Iron: 0.3, 0.55

    So the Earth is close to the largest planet possible with a room-temperature solid surface and 1 Earth-gravity surface acceleration of gravity.

    For a planet of pure water ice, one gets for 1 Earth mass 1.4 Re and 0.5 g's. Getting up to 1 g requires 4.8 Me and 2.2 Re.
  10. Sep 17, 2016 #9
    In the wiki page it is 2.34 and 5.6 times. Though on the main Gliese page some assclown listed the mass as 0.31 solar masses. How did you get 7g? Shouldn't it be 1.44g?
  11. Sep 17, 2016 #10
    that's interesting. do you know which are the candidates for the largest 1 g planets?
  12. Sep 18, 2016 #11
    g = ((G × mp) / R2) / 9.80665

    G = The Gravitational Constant = 6.674E−11 N⋅m2/kg2;
    mp = The mass of the exoplanet in kg (in this case I used 4.168E+25 kg, or 6.98 M); and
    R = The radius of Earth in meters (6.371E+06).

    The 9.80665 is Earth gravity at sea level in m/s2.

    It is only intended to be an approximate estimate, which is why I rounded it to the nearest one tenth.
  13. Sep 18, 2016 #12
    why did you use the Earth's radius instead of the exoplanet's radius?
  14. Sep 18, 2016 #13
    but wouldn't weakness prevent the planet from attaining a large size?
  15. Sep 19, 2016 #14
    You are absolutely correct. I should have used the radius of the exoplanet, not Earth. My mistake. That would clearly change the result to ~1.4 g, not the ~7 g I mistakenly posted previously. Thank you for pointing out my error.
  16. Sep 19, 2016 #15
    See the absurdity?
    Should be 1.00, 1.00 in both cases.
  17. Sep 20, 2016 #16
    Assuming the OP is referring to an exoplanet that is either within, or reasonably close to being within the "conservative" habitable zone of its parent star, then Kepler-62f might fit the requirements.

    According to Kopparapu et al.(2014) the "conservative" habitable zone for the star Kepler-62 is between 0.463 AU and 0.841 AU. Kepler-62f is 0.718 ± 0.007 AU from its parent star, has a radius that is 1.41 R, with an estimated mass of 1.843 M, and an estimated density of 1.3 ρ, giving it an estimated gravity of 0.927 g.

    It might be a little on the cool side, but theoretically (assuming an Albedo between 30% and 35%, a primarily nitrogen and oxygen atmosphere with similar atmospheric pressure, and greenhouse gases as on Earth) still close enough to its star to support liquid water on the surface. It is also smaller than 1.6 R, which increases its likelihood that Kepler-62f would be rocky.
  18. Sep 25, 2016 #17
    Nah, I don't care about that. :smile:

    All I'm looking for is a yuuugggeee planet that you can walk on (maybe requiring a protective suit) but doesn't make your backpack heavier than on Earth.
  19. Sep 26, 2016 #18
    Kepler-62f is still your best bet. You would be hard pressed to find another exoplanet smaller than 1.6 R, but larger than 1 R, and still be close to 1.0 g. Once you get to 1.6 R and larger, then you are reducing the chances of the planet actually having a surface upon which you can stand. Kepler-220c is 1.57 R, but it has a mass that is 4.092 M, which would give it 1.66 g.

    The ideal exoplanet to meet your criteria would have to have 1.599 R and 2.557 M.
  20. Oct 4, 2016 #19


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    I have removed some posts discussing traveling to such exoplanets. Lets stick to science and not move this thread into sci-fi.
  21. Oct 24, 2016 #20
    thanks. I didn't think it was necessary to address space travel. My initial question was and still is about the existence of hard, yuge planets of 1 g.

    A secondary question is whether gravity (of an exoplanet) is the most important factor since it is the one factor we cannot change.
  22. Oct 24, 2016 #21
    That is a question that cannot be answered ... yet.

    There is speculation that 0.378 gravity (Mars gravity) may be sufficient to support human life long term, but without studying the effects we have no way of knowing for certain. We do know that the long term effects of zero gravity is not conducive to human life. What we do not know is the minimum or maximum amount of gravity humans can endure in the long term and still function normally as on Earth. That is a question that cannot be answered with any certainty until we are in such an environment and can study our physiology.
  23. Oct 24, 2016 #22
    Well, Elon Musk will soon make that a reality. Hopefully.
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